Impressive feats without feet: the multi-scale physics of snake climbing

Some snake species can navigate diverse arboreal habitats with ease, propelling their limbless bodies over tree trunks and branches of varying diameter, orientation, flexibility, and surface roughness. However, other species struggle or even fail to climb. We take a multi-faceted approach and explore behavioral strategies that lead to success as well as skin adaptations that may enhance frictional interactions. For behavioral studies, we investigate the movement strategies for both frequent (corn snakes) and occasional (kingsnakes) climbers on our smooth vertical climbing wall, from which evenly spaced force-sensing protrusions and serve as the only “footholds”. Combining time-resolved 2D force data with 3D kinematics for vertical ascents and descents, we analyzed how body shapes and distributions of applied forces vary with the length and spacing of surface features. We found that corn snakes alter their behavior as conditions become more difficult (e.g., shorter or fewer footholds), but they were successful in a wide range of conditions. Inspired by these results, we built a simple robotic model and found that, when it was programmed to mimic a simple corn snake strategy observed in our experiments, it climbed easily. Surprisingly, our robot outperformed the kingsnakes, which climbed poorly in all conditions. To allow for broad parameter variation, we developed a mathematical model to compare the diverse strategies in the context of trade-offs between stability and speed. Examining small-scale skin adaptations, our lab has recently found that the structures present on the surfaces of snakeskin are unchanged by the preservation process that specimens undergo when added to a museum collection, allowing us much broader access to samples across the phylogenetic tree. Using this result, we examine the skin texture and textural variation in the context of potential adaptations for climbing. We observed longer spike-like features in preliminary AFM scans of the ventral scales of arboreal snakes, which we hypothesize may enhance lateral friction.